Abstract

Cableways and other structures that are used in connection with measurements of river discharge are designed for definite and specific purposes, and the various conditions that may affect their use require careful consideration. The selection of the site for a cableway should be made with due regard to the hydraulic conditions of the river as well as to the formation of the river banks. In current meter measurements, the direction and pattern of the stream flow and its possible effect on the performance of the current meter are of major importance. The channel should be straight at the place of measurement and the river should be flowing smoothly, without eddies or cross-currents. It is desirable that the flow should be in one channel at all stages; sections having overflow or diversions into old channels should be avoided if possible. Particles of drift in surface flotation that may be observed from the river banks or a boat should move in parallel straight lines and in a direction that is parallel with the river banks. The section should be free of large rocks and boulders; the bottom should be as smooth as is obtainable. It is always desirable to observe conditions at various stages of the river, especially at medium and high stages, before deciding upon the site.

The fundamental principles that underlie the proper use of the cur¬rent meter in making discharge measurements are fairly-well established, although the methods of procedure may differ with the various conditions that are encountered and with the available equipment. To obtain an accurate measurement of the discharge it is necessary to determine the depth at definite places in a cross section of the channel and to measure the velocity at each of those places. Therefore, the method of procedure should permit the placement of the current meter at definite and specific positions in the cross section.

Current-meter measurements are made from bridges or cableways, by wading, from ice cover, from boats, or from improvised structures in accordance with the circumstances and conditions at the place of measure¬ment. The general procedure to be employed in each of these types of measurement has been described elsewhere.

Because of the more satisfactory conditions of velocities and stream bed that are generally obtainable at sites selected for cableway instal lations, as well as the more convenient arrangements for the suspension and placement of the current meter, cableways are preferred to bridges or boats for rivers that can be safely spanned with ordinary types of struc¬tures. Outstanding examples of cableway construction are the 1 1/4 -inch galvanized tramway track-strand cableway with a clear span of 1,654 feet at the gaging station on the Columbia River at Trinidad, Wash. V, and the 1 3/8 -inch galvanized tramway track-strand cableway with a clear span of 1,727 feet at the measuring section on the Columbia River 9 miles upstream from Celilo Falls, Oreg., that is shown in the photograph.

First posted May 2010